Wheel hub drive systems are direct drives with which the wheel is driven without transmission, for example, in the form of a gear. Instead, the rotational speed of the motor and of the driven wheel are the same at every point in time.
Electric direct drives on the basis of permanently excited synchronous motors permit a very high efficiency so that the battery of the vehicle only has to store a fraction of the energy in comparison to the energy stored in the tank of a vehicle that has an internal combustion engine.
In the past, directly driving electric motors were already integrated into the vehicle wheel, whereby the stator and the rotor of the electric motor were connected to the stationary part of the wheel bearing or of the wheel hub. This translated into a space savings compared to centrally driving electric motors that drive several wheels at the same time.
A power converter in the vehicle comprises a power electronics unit that generates a multiphase alternating current from the direct current of the vehicle's own battery and uses this current to operate the electric motor. For the sake of simplicity, this is referred to below as the converter.
A problem in this context is that, for safety reasons, a friction brake also has to be provided and consequently, it has to be integrated into the rim space of the wheel together with the electric motor, in other words, especially with its stator and its rotor.
For this purpose, European patent specification EP 1380459 B1 has already proposed a wheel hub drive system which is based on the external-rotor motor principle (the rotor surrounds the stator radially on the outside) and in which a disc brake is arranged inside the electric motor.